Blocking is the unwanted adhesion between layers of plastic film that may occur under pressure, usually during storage or use. It is known that blocking can be prevented with the use of antiblocking agents which are added to the composition which makes-up the surface layer of the film. Known antiblocking additives for plastic packaging film include synthetic waxes. In U.S. Pat. No. 4,692,379, various antiblocking agents for a heat sealable outer skin of a multilayer film are specifically described, including silica, clay, talc and glass.
Sealable coatings used on flexible packaging films so that the films can be sealed with the application of pressure and with or without out exposure to elevated temperatures can pose blocking problems. A typical cold seal coating is a natural or synthetic rubber latex which tends to be tacky at room temperature and causes blocking. The rubber component permits sealing with slight pressure and without using heat. The cold seal coating is usually applied to a plastic film as it is wound into a roll. Since the cold seal coatings are tacky, it is important that the backside of the film which contacts the cold seal coating upon winding does not stick (block) to the cold seal coating so that the film can be easily unwound for use on packaging equipment.
One approach for reduced blocking between the cold seal coating and the backside of the film has been to formulate a cold seal coating which is nonblocking to certain surfaces including polypropylene, such a cold seal formulation is described in U.S. Pat. No. 5,616,400.
Another approach uses a cold seal release material on the layer opposite the cold seal surface, see U.S. Pat. Nos. 5,482,780; 5,489,473 and 5,466,734.
In U.S. Pat. No. 4,692,379, a film is described which has an upper heat sealable layer formed from an ethylene-propylene-containing copolymer or terpolymer and an antiblocking agent, the lower heat sealable layer is formed from an ethylene-propylene-containing copolymer or terpolymer and antiblocking agent and a quantity of silicone oil such that the coefficient of friction-reducing amount of the silicone oil will be present on an exposed surface of the upper heat sealable layer following mutual contact of the upper and lower surfaces. The silicone oil additive is described as having a viscosity of from about 350 to about 100,000 centistokes with about 10,000 to about 30,000 centistokes being preferred. An advantage of the invention as described in the '379 patent is that the silicone is present on the exposed surface of the lower layer in discrete microglobules which, to some extent, transfer to the upper surface upon contact. The silicone on the surfaces of the film facilitates machinability.
An attempt was made to produce a block-resistant functional film, typically a film having a printing function or sealing function, with silicone oil in a surface layer as an antiblocking agent. It was found that the silicone oil was detrimental to the printing or sealing function.